This invention relates to optoelectronic interface systems, and more particularly to methods and apparatus for conversion of information having high data rates between electrical and optical signals.
High speed data processing nodes require that information within the nodes, as well as between the nodes, be transmitted via data links that provide as great a data rate as possible. Limitations on the maximum permissible data rate of the data links include the limited information signal bandwidth and signal-to-noise ratio of the data link channel and the limited information signal bandwidth, signal-to-noise ratio and power dissipation of the data link translators.
According to the prior art, intra-mode data links and most inter-mode data links comprise translators that comprise transceivers or transmitters and receivers connected together via data link channels that comprise electrical signal transmission lines. Electrical translators generally have a trade-off between information signal bandwidth, signal-to-noise ratio and power dissipation. This is because the dimensions of the translators must be small to secure good information signal bandwidth, but the small size of translators with good information signal bandwidth limit their power handling or signal recovery ability, causing poor signal-to-noise ratio.
Electrical data link channels have a trade-off between information signal bandwidth and signal-to-noise ratio because much of the noise content of electrical data link is typically due to capacitive or inductive coupling of stray electrical signals or noise and the amplitude of such noise is generally proportional to frequency response of the data link channel. Although the data link channel may be made low impedance to reduce stray coupling, the higher signal current results in greater channel signal attenuation. This causes a loss in signal-to-noise ratio unless the data link translators can handle higher power levels, but this is usually not possible because the translators with good information signal bandwidth characteristics generally have limited power handling ability.
Optical signal data links can overcome many limitations of the electrical signal data link systems for both intra-mode and inter-mode data transmission. Optical data link channel noise is generally much lower than that of electrical data link channel noise due to insignificant optical stray signal coupling levels. The information signal bandwidth of optical data link channels is generally much better as well.
Although the performance of electro-optical signal translators can have advantages over electrical signal translators for data links both in terms of information signal bandwidth and signal-to-noise ratio, superior performance is hard to secure from such electro-optical signal translators because of the critical design parameters that are required for their fabrication. Amongst the problems that are encountered are efficient coupling of the electro-optical signal translators to the optical signal channels and alignment of the translator components to each other.
Each optical signal channel comprises a suitably terminated optical fiber. The electro-optical signal translators according to the prior art generally comprise a laser source element, a modulator element, a detector element, or a combination thereof, mounted on respective mounting substrates. For instance, an electro-optical translator that comprises a receiver module typically has a configuration that comprises a detector element mounted on a respective substrate.
Electro-optical translators that comprise transmitter modules generally have a configuration that comprises a laser source element mounted on a respective mounting substrate. The electrical power supplied to the power source input of the laser source element varies in intensity with the electrical input signal to directly modulate the laser source element. Although this configuration is simple, it generally suffers from poor output signal bandwidth and signal-to-noise ratio, since the input signal must be electrically amplified to a high level to modulate the laser source element and the laser source element has a limited signal frequency bandwidth.
All directly modulated lasers, except for those that employ distributed feedback, have serious color dispersion when coupled to a single mode fiber. This is because the wavelength of the laser hops around as its current is modulated. The different wavelengths that are generated as a result of mode hopping by the directly modulated laser are each propagated by a single mode optical fiber as a single mode. However, these different propagated wavelengths also have different velocities in the optical fiber, so that serious modulation noise and signal frequency bandwidth reduction result.
Although lasers that employ distributed feedback can be directly modulated without mode hopping, they are very expensive. Furthermore, all directly modulated lasers, even those that employ distributed feedback, suffer from relaxation oscillations, or ringing, when modulated at very high data rates. Ringing can cause serious modulation noise and signal bandwidth loss unless it is well above the highest modulation signal frequency of interest.
Another, less common, configuration for optical transmitter modules comprises a laser source element coupled to an electro-optical modulator element, with both the laser source element and the modulator element mounted on a common mounting substrate. This configuration can provide excellent output signal bandwidth and signal-to-noise ratio with a relatively small input signal. This is because the input signal and the modulator input are both low level, so that design parameters may be optimized for electro-optical signal conversion performance rather than power rating. However, this configuration requires accurate and efficient alignment between the laser source and modulator elements on their mounting substrates, as well as associated coupling and output optical fibers.
Still another transmitter module configuration that has been tried uses a laser source stage integrated with a modulator stage within a single element in an attempt to provide the performance of a transmitter module that has a separate modulator element but alignment that is simplified so that the alignment steps are limited to the same number as the configuration that uses only a laser source element. However, the attempts to manufacture such a transmitter module configuration to date have been unsuccessful.
The performance of the electro-optical signal translators depends as much upon the alignment of their component elements and the alignment of these elements with the corresponding optical channels as the performance of each individual element. Because of this, attempts to fabricate high performance electro-optical signal translators have been hampered by tedious and laborious "active" alignment techniques, wherein the electro-optical signal translator must be constantly tested in performance during assembly to achieve accurate alignment.
"Active" alignment techniques, that is, alignment techniques that require operational testing of the transmitter module during alignment, have been required for fabrication of transmitter modules having any of the configurations described above because of the difficulty in accurately manipulating, mounting, and bonding the associated transmitter elements and optical fibers. The active alignment of the transmitter module configuration that uses separate transmitter laser source and modulating elements to the optical coupling and output fibers on the mounting substrate is particularly costly and tedious, making the manufacture of such high performance transmitter modules very expensive.